1. Field of the Invention
The present invention relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to a power source that is particularly useful at elevated temperatures such as those found in well drilling operations as well as at very low temperatures down to −40° C., and lower.
In certain applications such as oil exploration, batteries are required that can operate at temperatures up to 200° C. and even higher. Batteries that are commonly used in such high temperature applications are based on oxyhalide chemistries, such as lithium/thionyl chloride cells or lithium/sulfuryl chloride cells. These cells are highly advantageous for high temperature use because the oxyhalide liquid cathode materials are stable at elevated temperatures. Furthermore, when pure lithium metal is used as the anode material, oxyhalide cells are useful at very low temperatures, down to −40° C., and lower, as well as at relatively high temperatures.
Regardless of the cathode chemistry, at about 180° C. lithium-based cells become extremely hazardous. Lithium metal melts at about 180° C. and can react violently with liquid catholytes or liquid electrolytes in solid cathode cell systems. The operating temperature of lithium can be extended upward somewhat through the use of certain alloys, for example, aluminum or magnesium. Since these lithium alloys have higher melting temperatures than pure lithium, cells having them as an anode material can be used at temperatures up to about 200° C., and higher. The drawback is that lithium alloys operate poorly at lower temperatures. Lithium alloy cells that can run safely at temperatures above 180° C. typically cannot deliver useful performance below about 70° C. This is a serious problem in most oil exploration applications where it is highly advantageous to test down-hole tools and their battery power sources at the surface under ambient conditions to confirm they will function properly when down-hole.
Accordingly, what is needed is a battery system that is functional at both ambient temperatures (25° C. or lower) and at temperature up to about 200° C. and higher. Such a battery system does not currently exist.
2. Prior Art
U.S. Pat. No. 4,461,815 to Peled et al. describes using calcium as the anode material in an oxyhalide cell. Calcium is a very attractive choice for use in an oxyhalide liquid cathode cell in some regards. First, it gives very high cell potentials close to what is obtained with lithium, and secondly, it has a much higher melting point than lithium. This means that calcium-containing cells are useful above 180° C. without becoming hazardous. However, calcium reacts with oxyhalide liquid cathode materials during cell discharged, thereby undergoing a self-discharge reaction that consumes active materials. This self-discharge reaction severely limits discharge efficiency, leading to low discharge capacity. Because of this severe limitation, calcium/oxyhalide cells have never come into practical use.